In my experience, low-back/high-butt pain usually comes down to three factors:

1. Posture/fit issues
2. Weak core muscles
3. Tight legs (especially hams, but could be any of the glutes, hip flexors, etc)

The yoga poses that help me the most are:
forward fold - for hamstring and back tightness
pigeon - for hip/butt muscles (I don't do the full pose as depicted, I just have my hands in front of me and my straight leg completely on the floor)
frog - hip/butt/inner thigh (sorry for the pornographic drawings on this one)
wide-leg forward bend - for inner thigh/leg tightness
reverse triangles - for IT band tightness.